1. Field of the Invention
The invention pertains to the field of optoelectronic devices. More particularly, the invention pertains to semiconductor light emitting devices.
2. Description of Related Art
There is a need in light sources providing emission with the controlled temperature dependence of the emission wavelength. In certain cases it is highly desirable to achieve a high stability of the wavelength of the emitted light with respect to the temperature of the device. This is very important for uncooled light emitting devices, in particular lasers, for applications in frequency conversion, atomic clocking, solid state laser pumping, wavelength division multiplexing and in other fields. In frequency conversion the wavelength of the laser must match the narrow spectral range for the efficient frequency conversion of non-linear materials at all temperatures of operation of the device. In atomic clocks, the lasing wavelength must pump the atomic transitions and should remain stable with respect to any temperature change. For pumping of solid state lasers, including fiber lasers, laser or light-emitting diodes with temperature-insensitive wavelength are needed. In wavelength division multiplexing multiple optical signals at different wavelengths are to be multiplexed into a fiber and separated at the exit of the fiber. Usually this is made by using diffraction gratings, which reflect light beams of different wavelengths at different angles. Thus, once the optical beams at different wavelengths are exiting from the same single mode and directed to the diffraction grating by means of necessary optics, they can be separated in the angle space and consequently measured separately without mixing of the channels. When the system is targeting wavelength multiplexing, the same diffraction effect helps to combine different beams together in a single fiber or a waveguide. If the wavelength is not stable the channels may be mixed, for example, as the diffraction angle changes as the wavelength shifts, the light spot also shifts and enters the next channel.
To achieve wavelength stabilization several approaches have been introduced. The most widely spread approaches are:                using etched and overgrown diffraction grating introduced into the waveguide region of the laser. Distributed grating-based Bragg reflectors create a narrow region of the optical feedback replacing or complementing the modes of the conventional Fabri-Perot laser cavity.        in another approach the gain medium of the device is placed into the vertical optical cavity of the surface-emitting device, for example, vertical cavity surface-emitting laser (VCSEL). The cavity is confined by Bragg reflectors providing a high reflectivity, which is necessary to achieve vertical lasing in a rather thin cavity. The device in this geometry may also operate as resonant-cavity light-emitting diode (RC LED).        
Other approaches include titled cavity laser, where the wavelength selection is realized via eptaxial layer sequence engineering of wavelength-selective losses, and tilted wave laser, where wavelength selectivity is realized by phase matching of the modes of two or more coupled waveguides with a different effective refractive index.
Even the wavelength of the emitted light in VCSELs and wavelength-stabilized edge emitters is fixed and the linewidth of the emission may be reduced dramatically, the wavelength stabilization is not complete. As the index of refraction changes with temperature, the resonant wavelength in the devices shifts. Furthermore, for the most of semiconductors, commonly used for market-relevant applications, the shift of the resonance features occurs as ˜0.6-0.7 nm/K. For the typical range of operation temperatures from −20° C. to 85° C. this results in thermal shifts of the order of 6-7 nm. Assuming also the need of the separating “windows” to avoid mixing of the signal between the channels the narrowest allowable spacing between the channels may be around 9-10 nm. For example, in case of WDM system operating in the spectral range of 840-860 nm standardized for data transmission in multimode fiber only two wavelengths can be placed without the danger of either mixing of the signals or getting the laser wavelength outside of the standardized spectral range.
Closer spectral spacing already requires temperature stabilization and monitoring, thus increasing the cost and power consumption of the device or assembly.
Similar problems exist in other applications. Thus, there is a need in a simple way of achieving temperature-insensitive wavelength-stabilized operation.
On the other side in applications in sensing and/or generation of terahertz radiation, a scanning of the lasing wavelength over a certain spectral range is needed. This scanning can be realized by temperature change which is affecting the refractive index. However, in this case, a significant shift may be achieved only by a strong heating of the device. The heating affects other characteristics, such as threshold current, differential efficiency and operation lifetime and may degrade severely the device performance.
Thus there is also a need in devices capable to significant tuning of the wavelength with only a moderate change in temperature.
It is also important to note that there exist materials which demonstrate positive or negative changes of the refractive index with temperature. For example, oxides typically show a shift of the resonance wavelength<0.3 nm/K, some of the oxides and the co-sputtered mixed oxide films show quasi-zero or even a strongly negative change of the refractive index with temperature (H. Hirota, M. Itoh, M. Oguma, Y. Hibino “Temperature Coefficients of Refractive Indices of TiO2-SiO2 Films” Japanese Journal of Applied Physics Vol. 44, No. 2, 2005, pp. 1009-1010). Other dielectrics, organic and composite films may also provide a wide range of opportunities to control the thermal shift of the refractive index.
However, there are limited options to implement the results in lasers as the maximum intensity of the optical field is concentrated in the active cavity of the device, wand the effective refractive index of the optical mode is determined by the host material.